powerpc: Cleanup memblock usage

* early_init_devtree(): Total memory size is aligned to PAGE_SIZE;
  however, alignment isn't enforced if memory_limit is explicitly
  specified.  Simplify the logic and always apply PAGE_SIZE alignment.

* MMU_init(): memblock regions is truncated by directly modifying
  memblock.memory.cnt.  This is incomplete (reserved array is not
  truncated) and unnecessarily low level hindering further memblock
  improvments.  Use memblock_enforce_memory_limit() instead.

* wii_memory_fixups(): Unnecessarily low level direct manipulation of
  memblock regions.  The same result can be achieved using properly
  abstracted operations.  Reimplement using memblock API.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Yinghai Lu <yinghai@kernel.org>

arch/powerpc/mm/init_32.c

@@ -134,7 +134,7 @@ void __init MMU_init(void)
 
 	if (memblock.memory.cnt > 1) {
 #ifndef CONFIG_WII
-		memblock.memory.cnt = 1;
+		memblock_enforce_memory_limit(memblock.memory.regions[0].size);
 		memblock_analyze();
 		printk(KERN_WARNING "Only using first contiguous memory region");
 #else